Embodiments of the disclosure relate generally to superconducting magnets and more particularly relates to a quench protection circuit for superconducting coils of the superconducting magnets.
In theory, superconducting magnets conduct electricity without resistance as long as the magnets are maintained at a suitably low temperature, which is referred to as “critical temperature” of the superconductor herein after. Accordingly, when a power source is initially connected to the superconducting magnet coils for a period to introduce a current flow through the magnet coils, the current will continue to flow through the coils after the superconducting switch is closed and power supply is disconnected because of the absence of electrical resistance in the coils, thereby maintaining a strong magnetic field in, for example, magnet resonance imaging (MRI) systems, and generators.
Cooling systems are used for maintaining the superconducting magnets below the critical temperature by, for example, immersing the superconducting coils in liquid helium, or by arranging other cooling apparatus such as cooling tubes thermally coupled to the superconducting coils to remove heat from the coils. A vacuum vessel and a thermal shield are provided for receiving the superconducting coils and minimize the convection and radiation heat load from ambient to the superconducting coils that should be maintained below the critical temperature. However, the magnets or part of the superconducting coils still may become normal (no longer superconducting) and develop a resistance that causes current flowing through the coils to decay rapidly converting the stored magnetic energy into Joule I2R heat that raises the temperature of the region. This is an irreversible action known as “quenching” or “quench”, which causes undesirable heat that can lead to increased temperature. The entire magnet can then become normal and no longer be superconducting. In addition, quench can lead to overheating within the superconducting coil or voltage spikes and arcing damage to components as well as the magnet. It is therefore desirable that quench protection devices can quickly spread the normal zone to the other portions of the coils and dump the magnetic energy into joule heat more evenly across the entire coils or magnet. This quench protection can limit the maximum temperature and voltage in the superconducting coil to be within the safe range, prevent any coil damage caused by over-heating, over-voltage, or over-stress.
One conventional quench protection apparatus includes a set of electrical quench heaters. When a quench occurs, temperature of the heaters arises quickly, and the heaters transmit heat to a larger area of the superconducting coils. The quench protection apparatus also includes a current limiter connected in series to the heaters for limiting the current flowing through the heaters to protect the heaters from overheating. Conventional current limiters, such as positive temperature coefficient resistors, are usually arranged outside the vacuum vessel. Accordingly, there are electrical wires extending through the vacuum vessel, which requires additional cost and might adversely decrease vacuum reliability.
It is desirable to have a different superconducting magnet with a simpler quench protection circuit.